1. Field of the Invention
The present invention relates to sensors and security systems. More particularly, the present invention relates to providing immunity to bright white light for Passive Infrared (PIR) motion sensors.
2. Related Art
PIR sensors are increasingly being used in numerous security and building automation systems. Automatic light switches, motion detectors, or remote control of electrical appliances are among these uses. PIR motion sensors can detect the infrared light emanating from an intruder, such as a human or motor vehicle, and transmit a signal across the security system. This signal can also be used to trigger an alarm.
PIR motion sensors detect motion based on the infrared (IR) energy emitted by known temperature ranges of the objects that need to be detected, for instance, a human body. The human body generates radiation in the wavelengths of 7 μm to 14 μm generally. To detect the human body, the motion sensor is equipped with a light filter that passes the desired wavelengths of 7-14 μm while reflecting and/or absorbing the rest. The energy that is absorbed by the filter causes heating of the filter. This heat is reradiated and is detected by a plurality of pyroelectric sensing elements within the sensor.
A pyroelectric sensing element responds to incoming radiation by generating an electrical signal that is amplified and processed. The signal represents the temperature of the object relative to a background temperature. The signal generally has two major components that are considered when being processed: a baseline, and a signal threshold (see prior art FIG. 1). When a heat source, such as a human body, moves in front of the sensor, it will cause the signal to vary away from the baseline. If the signal is sufficient to cross the threshold, an alarm may be triggered.
One problem with PIR sensors is that visible light can trigger the pyroelectric element of the sensor. PIR sensors generally have an inherent vulnerability to bright white light, for instance car headlights or bright flashlights. Specifically, when a bright white light is shined on the pyroelectric element, it introduces a signal in this element that, when amplified, will seem to be a legitimate target to the processing component of the sensor. Prior art FIG. 1 illustrates the response of a pyroelectric sensor when bright white light reaches the sensor, such as car headlights shined through a window. The upper graph 10 illustrates a step change of the intensity of the light reaching the sensor and the lower graph illustrates a pyroelectric response signal 20 from the pyroelectric sensor, as well as an upper threshold 22 and a lower threshold 24. As shown, a pyroelectric signal deviating from baseline 26 due to the bright white light exceeds the upper threshold 22 and an alarm is issued.
Thus, a bright light can push the PIR signal over the threshold and trigger an alarm. These false alarms are problematic and reduce the reliability of the sensors. Further, this inherent vulnerability provides a means for a malicious person to force a motion detector into alarm from outside a building by shining a bright light in the direction of the sensor.
Traditional approaches to solving this issue include augmenting the ability of the pyroelectric detector window/filter to block unwanted wavelengths. Pigmentation such as Zinc Sulfide added to the lens blocks white light while passing infrared energy. Alternatively, secondary optical filters can be added. Both approaches have their drawbacks and neither approach satisfies the desire for significantly improved, if not unlimited white light immunity.
Another method involves temporarily increasing the alarm threshold to desensitize the unit in the presence of a bright white light. A secondary light sensor is utilized to detect when lighting conditions change. If the change in light intensity exceeds a predetermined threshold for this light sensor, any potential alarm signal is suppressed for a predetermined time period of this change. This will prevent false alarms when a light is shined on the motion detector. However, this also drastically decreases the sensitivity of the sensor, making it prone to spoofing, i.e. a person using a bright light to desensitize the motion detector, and walking through the detection zone undetected.
Accordingly, it would be advantageous to provide a motion detecting method and system that provides significantly improved white light immunity while obviating intentional blinding of the sensor by an intruder.